docs: def _get_stddev_deep_soil(self, mag, imt):
          """
          Calculate and return total standard deviation for deep soil sites.
  
          Implements formulae from the last column of table 4.
          """
          # footnote from table 4 says that stderr for magnitudes over 7
          # is equal to one of magnitude 7.
          if mag > 7:
              mag = 7
          C = self.COEFFS_SOIL[imt]
          return C['sigma0'] + C['magfactor'] * mag